Coxiella burnetii is an obligate intracellular gram-negative bacterium that causes the worldwide zoonotic disease, Q fever. Natural human infections commonly occur via inhalation of infectious aerosols, manifest clinically as atypical pneumonia, and can develop into a severe, fatal chronic disease. C. burnetii is an understudied category B select agent, and there is no licensed vaccine in the US. Creation of a safe and effective vaccine for preventing human Q fever is an important public health and national biosecurity goal. The long-term goal of this program is to develop new approaches to discovery of safe, effective vaccines to aerosol-transmitted intracellular bacterial pathogens. The objective of this application, which is an important step toward this goal, is to determine the role of antibody (Ab)-mediated immunity to lipopolysaccharide (LPS) in protection against pulmonary infection with aerosolized C. burnetii. To achieve this objective, we will test the central hypothesis that immunoglobulins that bind to C. burnetii phase I LPS (PI-LPS) play an important role in protecting vaccinated hosts against Q fever. This hypothesis is based on observations that i) passive transfer of Ab purified from C. burnetii phase I vaccine (PIV)-immunized mice conferred protection against C. burnetii infection in naive recipient mice, and ii) PI-LPS conferred protection against C. burnetii challenge. To test this hypothesis, this proposal focuses on identifying antigens targeted by protective Ab and understanding the mechanisms of Ab-mediated protective immunity against aerosolized C. burnetii. The proposal has two specific aims: Aim 1: to identify antigens targeted by protective Ab. We will identify the protective Ab-recognized antigens and determine if C. burnetii PI-LPS is the key protective antigen responsible for PIV-induced protection. Aim 2: to determine the mechanisms of Ab-mediated protection against pulmonary C. burnetii challenge. We will determine i) if protective Ab activates complement and thus lysis of extracellular C. burnetii, ii) if Ab-mediated protection depends on Fc receptor-mediated effector functions, iii) if Ab-opsonized C. burnetii enhances macrophage phagocytosis, iv) IFN-? is required for Ab-mediated clearance of intracellular C. burnetii and v) if Ab-mediated protection requires help from T cells and to identify the T cell types that are important for Ab-mediated protection. As an outcome of this research, we expect to determine if PI-LPS is a target in protection against Q fever and to understand the contribution of humoral and cellular immunity in vaccine-induced protective immunity against C. burnetii infection. This is expected to have significant positive effects on human (and animal) health, because it will provide information for vaccine design and Ab-based immunotherapeutic strategies for Q fever, and in turn lead to development of new strategies to interfere with aerosol transmission of other dangerous intracellular bacterial pathogens. PUBLIC HEALTH RELEVANCE: Coxiella burnetii is an understudied category B bioterrorist agent that causes acute Q fever and chronic infections in humans. To fill the fundamental gap in knowledge regarding the mechanisms of protective immunity and to develop a safe and effective vaccine against Q fever, this proposal focuses on identifying the antigenic targets that recognize the protective antibody and understanding the mechanisms of antibody-mediated protective immunity against C. burnetii aerosol infection.